How To's

Ask The Experts: Battery Charging Rates

Is there some sort of a standard maximum charge rate for flooded lead acid batteries? Someone mentioned not wanting to take them beyond a C/10. What’s the rationale for that? I’d rather hit them hard and fill them up quickly while my generator is running. Thanks,

John Bartlett • Exeter, New Hampshire

Hello, John. The maximum recommended rate is C/5 (a charge rate in amps of one-fifth the overall battery capacity in amp-hours), but only when the cells are between 10 and 85 percent state of charge (SOC). After the cells reach 85 percent SOC, then a C/10 is the maximum. After cells reach 95 percent SOC, between C/20 and C/15 is recommended. Having said this, I rarely charge faster than a C/10.

The reason for the maximums is heat. Higher amperage means more heat, particularly when the cells are getting fully recharged. Thermal cycling wears the plates and sloughs off material.

Finally, it’s always a good idea to get charging specifications from the manufacturer of the particular model of battery you’re running. Battery specific charge rates, and bulk, absorption, float and equalization set-points and times will all lead to better battery longevity.